Database
[{exchange_order_products:[]},{exchange_order_products:[{}]}]
Query
db.collection.aggregate([{$group:{_id:null,empty:{$sum:{$cond:[{$eq:[{$size:"$exchange_order_products"},0]},1,0]}},nonEmpty:{$sum:{$cond:[{$eq:[{$size:"$exchange_order_products"},0]},0,1]}},}},{$project:{percent:{$multiply:[100,{$divide:["$nonEmpty","$empty"]}]}}}])